Press C to Crawl
(but you might have to rebind that)
This mod is very simple, It allows a player to press (by default) "C" to be put into the new crawling position that was added, In my opinion this feature felt underused, and had no friendly way of activating it, so this mod is to counter that.
you can change the crawl bind in your controls menu, it is in its own category "Crawl"
There is config, but it only has one thing in it: by default when you press your crawl key it will force you down into position using an invisible block (this block disappears eventually naturally or can be insta-mined or jumped on) but if you continue crawling in open air, it will not keep you down. To enable to ability to "crawl in open terrain" you can enable "shouldContinue" in config, this defaults to false as the blocks can sometimes take a while to naturally disappear
you can report any issues on the github page.
you can of course use this mod in modpacks
please don't repost this mod anywhere without my permission in the comments
Note: you can also force someone into a crawling position by giving them the "crawling" scoreboard tag, this might be useful for map makers etc
If you found this mod first, you should go check out GoProne which does almost exactly the same thing (I didn't realise at the time) and uses packets instead of blocks so might be better for different people